The temperature of a produced hydrocarbon containing stream at subsea level is generally hot and may normally vary in temperatures in the range of 30-150° C. at the prevailing pressures. In order to process or transport such a hot hydrocarbon fluid stream, it may be necessary to regulate the temperature of the stream, such as by cooling the stream to a specific temperature, e.g. in the range of 0-60° C.
A number of existing oil coolers located on the seabed are not controllable and have the limitation of only cooling the oil from a well stream after separation of gas the phase. Such coolers are used to reduce the temperature of the oil either because of pipeline temperature restrictions or for other reasons.
There is also a small number of gas coolers installed subsea.
A known type of subsea coolers consists of bundles of steel tubes in which the warm process stream flows. Seawater at approximately 4° C. freely flows across the outside of the tubes, directly cooling the process stream by heat dissipating through the steel tube walls to the surrounding seawater. The amount of cooling strongly depends on the sea current, as the heat transfer is increased multiple times if there is a strong sea current. In this way, the outlet temperature of the process stream is given by uncontrollable variables. For some applications where any additional cooling is considered an advantage, this may be satisfactory.
However, hydrocarbon fluids may develop wax and hydrate depositions when cooled down below certain temperatures. A build up of wax and/or hydrate depositions inside tubes or process equipment will reduce the capacity of said parts and in worst case lead to blocking which stops productions and is both time consuming and costly to remove. Hence, uncontrolled cooling of hydrocarbon fluids at subsea is undesirable, as the outlet temperature of such coolers may result in undesirable process conditions, such as temperatures below the wax or hydrate formation temperature in the fluid.
In a more elaborate subsea processing system than oil cooling or gas compression, e.g. for future controllable water knock-out or hydrocarbon dew-point control, the outlet temperature of a subsea heat exchanger needs to be controllable. This will enable the temperature of the outlet process stream to be fixed, independently of changing flow streams, flow temperatures, sea temperature and sea current.
A temperature control of hydrocarbon containing process streams will also be advantageous for hydrate control as there is a risk in subsea systems of having areas cooled down below hydrate temperature (typically 20-30° C.). To avoid deposits such as hydrates, below said temperatures, hydrate inhibitors are added to the stream. Conventional hydrate inhibitors may be an alcohol (methanol or ethanol) or a glycol such as Monoethylene Glycol (MEG or 1,2-ethanediol), which is inexpensive and simple to inject. However, the amount of hydrate inhibitor needed is proportional to the water content of the stream and at high levels will require a hydrate inhibitor regeneration process at the receiving end.
By controlling the flow of water across the tube bundle carrying a hydrocarbon containing stream, control of the outlet temperature of the cooler might be provided. The current suggested manner to do this is to enclose the tube bundle in an enclosure (large tank, shell or jacket) with openings in each end for through-flow of seawater. The amount of seawater entering/exiting the tank, i.e. the seawater flow across the tube bundle, may be controlled in order to adjust the heat transfer coefficient and hence the outlet temperature of the hydrocarbon-containing stream.
WO 2008/147219, (FMC Kongsberg Subsea), concerns a subsea cooling unit having an inlet for a hot fluid and an outlet for cooled fluid, the cooling unit comprising a number of coils exposed to seawater, and means for generating a flow of seawater past the coils, where the means for generating the flow of seawater comprises a propeller and a rotatable actuator and wherein the cooler is enclosed in a duct.
WO 2010/002272, (Aker Subsea AS), concerns a subsea convection heat exchanger for cooling or heating a hydrocarbon-containing fluid in subsea environment. The heat exchanger comprises a convection section with a fluid carrying pipe adapted for heat transfer between the carried hydrocarbon fluid on one side of the pipe wall and the surrounding water on the opposite side of the pipe wall. The convection section is enclosed by an enclosure with a seawater inlet and a seawater outlet and the heat exchanger is provided with means for controlled through-flow of the surrounding seawater from the seawater inlet to the seawater outlet.
Hence, the documents above describe a control mechanism for adjusting the seawater flow across the tube bundle using a propeller or a pump respectively.
U.S. Pat. No. 3,158,010 concerns a heat exchanger with helically coiled tube bundles.
The use of seawater for the direct cooling of tubes carrying the relative hot hydrocarbon containing fluids can give rise to unwanted scaling and marine growth or bio fouling. Scaling may occur on surfaces of high temperatures in seawater, such as 60° C. and above, by deposition of inorganic compounds, such as CaCO3 and CaSO4. Bio fouling or growth may occur on any subsea equipment, but is enhanced by warm temperatures, such as on the warm tubes of a heat exchanger. In addition, high temperatures increase the risk of corrosion of the cooling tubes, which may lead to leaks and malfunction.
Scale formation and fouling on tube walls reduces the efficiency of the heat transfer by the formation of an insulating film or layer on the seawater side of the tube surfaces and may in addition induce corrosion. The formation of such layers not only reduces the heat transfer and the heat exchanger efficiency, but may also narrow the passage way for the sea water flow. These effects may both call for increased seawater through put, such as by increased pump capacity or an over-sizing of the heat exchanger to compensate for a reduction in total heat transfer. Alternatively a regular cleaning of the surfaces exposed to such scaling and fouling may be needed.
The heat exchangers suggested in prior art with controllable output temperature are subject to the mentioned scaling and fouling. Due to the encasements of the tube bundles, wherein seawater is contacted with the warm hydrocarbon carrying tubes, are difficult or impossible to clean physically. In addition, the use of biocides and/or anti-scale chemicals for removing such scaling and fouling is unwanted due to the difficulty in handling such substances at subsea level in safe and controlled fashion with regards to environmental concerns.
Hence there is a need for a subsea heat exchanger with output temperature control, which is not subject or less prone to the concerns of the heat exchangers of prior art.